The present invention relates generally to X-ray tubes, and more particularly to a target assembly for an X-ray tube and to a method for making the target assembly.
X-ray equipment used in the medical field typically includes a rotating anode X-ray tube. Such X-ray tubes are vacuum tubes whose anodes each include a rotor having a rotatable rotor shaft and also include a stator which circumferentially surrounds, or is circumferentially surrounded by, the rotatable rotor shaft. A pair of bearings, such as rolling element bearings (e.g., ball bearings), is positioned radially between the rotor shaft and the stator.
The anode also has an X-ray target which includes a target cap attached to an annular target shaft. The target cap has a target track portion which produces X-rays and heat when struck by electrons emitted by the X-ray tube's cathode. The target cap may also have a central bore used to evacuate the anode when creating the vacuum during tube construction. The target typically also includes a graphite heat sink attached to the target cap. The target shaft is bolted to the rotor shaft either directly or through intermediate members. The bearings get heated to high temperatures since some of the heat produced by the target track region of the target cap flows by solid conduction from the target cap to the target shaft and then from the target shaft to the bearings. The bearings are poor thermal conductors which sets up a temperature differential between the rotating side and the stationary side of the bearings causing bearing misalignment and wear which shortens the operating life of the X-ray tube. Additional heat also comes from backscattered electrons entering the bore of the target cap and impinging on anode structure near the front bearing.
Known designs include X-ray-tube targets which have been constructed by bolting, or threading and brazing, together two target shaft members. Such bolted, or threaded and brazed, joints are under cyclic thermal and mechanical loads and have become loose over time causing tube vibrations requiring immediate system shutdown and tube replacement.
What is needed is an improved X-ray-tube target which reduces heat flow to the bearings without having a tendency to cause tube vibrations requiring unscheduled system shutdown for tube replacement.